Whereas, in a conventional inkjet printer, the print heads which are mounted on a carriage spray ink droplets line by line in the transverse direction (also called X-direction) onto the medium which is transported discontinuously in the running direction (also called Y-direction), in a single-pass inkjet printer the print heads are mounted in print head modules in the transverse direction over the entire width of the medium. The printing medium can be moved continuously in the running direction. Whereas printing speeds of up to 2 m/min are achieved in a conventional inkjet printer, printing speeds of up to 50 m/min can be achieved by way of a single-pass inkjet printer. For color printing, a plurality of print head modules are mounted one behind another in the running direction in a single-pass inkjet printer. Here, the print head modules are assigned in each case one primary color, in particular cyan, magenta and yellow, and possibly black. For special printing uses, further print head modules with a special color can be added.
A single-pass inkjet printer is suitable, in particular, for industrial use, in which bulk articles have to be printed and a high throughput is therefore important. A single-pass inkjet printer is likewise suitable for printing large-area objects on account of the high printing speeds. A single-pass inkjet printer is therefore suitable, in particular, for industrial applications of the furniture or ceramic industry, where floor coverings, such as laminates or ceramic tiles, worktops, moldings or the like are to be provided with a decorative pattern. Here, a very wide variety of inks are used which are, for example, resistant with respect to a later protective covering, etc.
In comparison with conventional printing processes, such as gravure printing or the like, the single-pass inkjet printer is used precisely even in the case of small batch sizes, where the production of an impression roll is not worthwhile. In contrast, a single-pass inkjet printer also makes individualization of the decorative patterns possible, and what are known as impossible decorative patterns which cannot be achieved by way of rolls. The single-pass inkjet printer is not restricted to a continuous repetition of a printing pattern or repeating pattern, as is the case in rotary printing.
An individual print head module for a single-pass inkjet printer can certainly achieve dimensions in the transverse direction and vertically (also called Z-direction) of more than half a meter up to over a meter. The print heads which are combined in the printing bars of a print head module can in each case have widths of up to several tens of centimeters. Here, resolutions of up to 600×600 dpi (dots per inch) are possible. Here, several thousand nozzles are contained per print head. Printing widths of up to a few meters can be achieved by way of large print head modules or by way of a plurality of print head modules being arranged next to one another.
Positional deviations of a few micrometers can be detected in a printed image by way of the human eye. In the case of the abovementioned resolutions, the individual nozzles of a print head lie only a few tens of micrometers apart from one another. The size of an image dot itself is in the range of 10 micrometers. It becomes clear that, in the case of a single-pass inkjet printer having a plurality of print head modules which are arranged one behind another in the running direction, an adjustment of the print heads in the micrometer range becomes necessary, in order to produce a high quality printed image. The adjustment of a print head module in a single-pass inkjet printer is therefore very complicated. For example, the position of the print heads which are mounted in the print head module has to be detected by light microscopy and set manually in a complex manner to this end. The setting up of a single-pass inkjet printer is therefore comparatively protracted.
With respect to the construction of a single-pass inkjet printer which is simplified with regard to the adjustment of the print heads, WO 2005/108094 A1 proposes to hold the individual print heads in each case in a prestressed state in a frame of the print head module. Here, each print head is pressed in its corresponding cutout against the opposite frame edge by means of a mechanical spring element. A prestress of this type can be performed both in the X-direction and in the Y-direction.
Disadvantageously, the position of the print heads with respect to the respective print head module is fixed in an arrangement of this type on account of the prestressed stop. Production-induced tolerances of the print head or the frame dimensions cannot be corrected in this way. Relative positioning of the print heads with respect to the print head module is not made possible.
Secondly, it is described in WO 2005/108094 A1 to mount the print heads in a print head module such that they can be displaced longitudinally in the transverse direction (X-direction) and in the running direction (Y-direction) in order to compensate for production tolerances. The position of a print head with respect to the print head module can be set by way of corresponding displacement of the print head. The relative orientation of the print heads with respect to the print head module takes place by means of a suitable tool, before the print head module with the print heads which are then positioned exactly is inserted in a positionally fixed manner into the single-pass inkjet printer.
It has been shown disadvantageously that a print head which is installed in a positionally fixed manner with respect to the print head module is capable of changing its relative position with respect to the print head module in the case of thermal cycling. Surprisingly, permanent positional changes of the print head can occur here, which lead to visible deviations in the printed image.